A magnetic random access memory (MRAM) is a non-volatile memory that is hopeful from the viewpoints of a high integration and a high speed operation. As a method of writing data to the MRAM, conventionally, an “Asteroid method” (for example, U.S. Pat. No. 5,640,343) and a “Toggle method” (for example, U.S. Pat. No. 6,545,906, Japanese Patent Application Publication JP-P2005-505889A) are known. According to these write methods, a switching magnetic field required to switch magnetization of a free layer becomes greater in substantially inversely proportional to a memory cell size. In short, there is known a tendency that a write current increases as the memory cell is made smaller.
As the write method that can suppress the increase of a write current in association with a fine structure, there are proposed a “Spin Transfer Method” (for example, Japanese Patent Application Publication (JP-P2005-093488A), and “Research Trends in Spin Transfer Magnetization Switching” (Japanese Applied Magnetic Academic Society Journal, Vol. 28, No. 9, 2004) by K. Yagami and Y. Suzuki. According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor, and the magnetization is switched by the directly interaction between a spin of a conductive electron as a carrier and magnetic moment of the conductor (hereinafter, to be referred to as a spin transfer magnetization switching). The outline of the spin transfer magnetization switching will be described below with reference to FIG. 1.
In FIG. 1, a magneto-resistance element contains a free layer 101, a pinned layer 103, and a tunnel barrier layer 102, which is a non-magnetic layer and put between the free layer 101 and the pinned layer 103. Here, the pinned layer 103 whose magnetization orientation is fixed is formed to be thicker than the free layer 101 and functions as a mechanism (spin filter) for generating the spin-polarized current. A state in which the magnetization orientations of the free layer 101 and the pinned layer 103 are parallel is correlated to a data “0”, and a state in which they are anti-parallel is correlated to a data “1”.
The spin transfer magnetization switching shown in FIG. 1 is attained by a CPP (Current Perpendicular to Plane) method, and a write current is vertically injected to a film surface. Specifically at a time of a transition from the data “0” to the data “1”, a current flows from the pinned layer 103 to the free layer 101. In this case, electrons having the same spin state as the pinned layer 103 serving as a spin filter move from the free layer 101 to the pinned layer 103. Thus, the magnetization of the free layer 101 is switched through a spin transfer effect (exchange of a spin angular motion amount). On the other hand, at a time of transition from the data “1” to the data “0”, the current flows from the free layer 101 to the pinned layer 103. In this case, electrons having the same spin state as the pinned layer 103 serving as the spin filter move from the pinned layer 103 to the free layer 101. The magnetization of the free layer 101 is switched through the spin transfer effect.
In this way, in the spin transfer magnetization switching, the write of data is carried out through the movement of the spin electrons. The magnetization orientation of the free layer 101 can be defined in accordance with a direction of spin-polarized current that is vertically injected to the film surface. Here, a threshold at the time of write (magnetization switching) is known to depend on a current density. Thus, as a memory cell size is contracted, the write current necessary for the magnetization switching decreases. In association with the finer structure of the memory cell, the write current decreases. Thus, the spin transfer magnetization switching is important in attaining the larger capacity of the MRAM.
As a related art, U.S. Pat. No. 6,834,005 discloses a magnetic shift register that uses the spin transfer. This magnetic shift register stores a data by using a domain wall inside a magnetic substance body. In the magnetic substance body with many regions (magnetic domains), the current is injected to pass through the domain wall, and the domain wall is moved by the current. The magnetization orientation in each domain is used as a record data. Such a magnetic shift register is used to record, for example, a large quantity of serial data. It should be noted that the motion of the domain wall inside the magnetic substance body is also reported in “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires” (PRL, Vol. 92, pp. 077205-1-0077205-4, 2004) by A. Yamaguchi, et al.
As a related art, Japanese Patent Application Publication (JP-P2006-73930A) discloses a method of changing a magnetization state of a magneto-resistance effect element by using the domain wall motion, and a magnetic memory element and a solid magnetic memory using the method. This magnetic memory element has a first magnetic layer, a middle layer and a second magnetic layer, and records data as the magnetization orientations of the first magnetic layer and the second magnetic layer. In this magnetic memory element, the magnetic domains that are magnetized anti-parallel to each other and the domain wall that separates those magnetic domains are steadily formed inside at least one magnetic layer, and the domain wall is moved inside the magnetic layer. As a result, the positions of the magnetic domains adjacent to each other are controlled for data recording.
As a related art, Japanese Patent Application Publication (JP-P2005-191032A) discloses a magnetic memory device and a method of writing a data. This magnetic memory device includes a conductive magnetization fixed layer to which a fixed magnetization is given, a tunnel insulating layer formed on the magnetization fixed layer, a conductive magnetization free layer that has a junction section formed on the magnetization fixed layer through the tunnel insulating layer, domain wall pinned sections formed adjacently to a pair of ends of the junction section, and a pair of magnetization fixed sections which are adjacent to the domain wall pinned sections and to which the fixed magnetizations of orientations opposite to each other are given, and a pair of magnetic data write terminals that are electrically connected to a pair of magnetization fixed sections, and are provided to flow to the magnetization free layer, a current which penetrates through the junction section of the magnetization free layer, a pair of domain wall pinned sections and a pair of the magnetization fixed sections.
As a related art, Japanese Patent Application Publication (JP-P2005-150303A) discloses a magneto-resistance effect element and a magnetic memory device. This magneto-resistance effect element has a ferromagnetic tunnel junction that has a 3-layer structure of a first ferromagnetic layer/a tunnel barrier layer/a second ferromagnetic layer. The first ferromagnetic layer is greater in coercive force than the second ferromagnetic layer. A tunnel conductance is changed in accordance with the relative angle of the magnetizations between the two ferromagnetic layers. This magneto-resistance effect element is characterized in that the magnetization of the end of the second ferromagnetic layer is fixed to a direction having a component orthogonal to a magnetization easiness axis direction of the second ferromagnetic layer.